Method for producing a microlens array

ABSTRACT

A method for creating microlenses, the method includes the steps of providing a substrate having a plurality of photoactive areas; providing a photopolymerizable fluid composition on the substrate; providing a template which is transparent to photoactive wavelengths and which includes a plurality of curved surfaces that act to focus incident light onto the photoactive area; placing the template on the photopolymerizable fluid composition which causes the fluid material to spread and substantially fill the curved surfaces of the template; and irradiating light through the template and onto the photopolymerizable fluid composition for hardening photopolymerizable fluid composition into microlenses spanning the substrate.

FIELD OF THE INVENTION

This invention relates to the fabrication of microlens arrays on thesurface of electronic image sensors.

BACKGROUND OF THE INVENTION

There continues to be a push to produce ever-higher resolution inelectronic image sensors. This means the surface area of individualpixels is becoming smaller and thus the amount of light impinging oneach pixel is also decreasing. Advances in the technology of theunderlying electronic image sensor have helped boost the signal-to-noiseratio to compensate for the decreasing amount of light. However, sincethe photoactive part of a pixel is often only 50% or less of the totalpixel area, the fabrication of microlens arrays aligned to the pixelarrays has proven to be a very effective way of increasing the fractionof incident light striking the photoactive area of the pixels.

A very efficient and manufacturable method of producing microlens arrayshas been to form patterns in photoresist materials by standardmicrolithographic techniques. These patterns, which would have squaredcorners upon formation, are then melted and thus rounded into microlensfeatures. A major requirement of this technique is that the individualmicrolenses need to be far enough apart so that when melted the adjacentpatterns do not touch. If they were to touch the patterns would flowtogether and not produce the desired microlens shape. Thus, thistechnique always results in gaps between adjacent microlenses. Thesegaps would be around the periphery of the pixels and any light impingingon them would not be focused onto the photoactive area of the pixel.Thus there is a need for an effective method to produce microlens arrayswhere these gaps are reduced or eliminated.

Progress has been made in the reduction of these gaps by severalmethods. A common practice in the art of microlens array production isto first form the microlens shape in an upper layer using the flowtechnique described above. This microlens shape is then transferred intoa lower layer by reactive ion etching (RIE). RIE is a plasma etchingtechnique whereby the reactive ions of the plasma are acceleratedtowards the substrate by an electrical bias. This causes a veryisotropic etch and an accurate transfer of the microlens shape into theunderlying layer. This technique is used when the microlens materialdoes not have the appropriate characteristics to allow directmicrolithographic patterning and melting. U.S. Pat. No. 6,163,407,discloses this technique to reduce the final microlens gap. This isaccomplished by altering the RIE conditions such that there is amismatch in the etch rates of the two layers. This results in a slightlydifferent microlens shape than the initial melted photoresist. Judiciousadjustment of the etch rates can result in smaller gaps in the finalmicrolens array. Although this method does result in reduced gaps, thereisn't enough process latitude to completely eliminate the gaps aroundthe entire perimeter of the pixel. Other disadvantages of this methodinclude the need for the extra pattern transfer RIE step and the risk ofdamage to the underlying image sensor from the plasma environment.

Another method holding promise for the production of gapless microlensarrays is gray scale lithography. This method involves patterning thephotoresist with a mask having a range of densities instead of thecommon 0% or 100%. The range of densities results in a range ofsolubilities of the exposed photoresist film. Thus the final photoresistprofile after development matches the light intensity distributiontransmitted by the mask. This method has several drawbacks however.First, as should be obvious, the design and production of the mask isquite complicated and expensive. Next, the photoresist must be able toaccurately reproduce the varieties of light intensities. This is bestaccomplished with a photoresist having a contrast around 1. These typesof photoresists are difficult to find since most photoresist developmentwork has been aimed at the high contrast needed to produce thehigh-density circuits used in modern electronic devices. Also, thesephotoresists most likely do not contain the characteristics necessaryfor use as the final microlens material. These include transparency tovisible light, stability to heat and light, and relatively highrefractive index. This means that the photoresist pattern needs to betransferred into an underlying layer similarly to the method describedpreviously. For these reasons gray scale lithography is not viewed as amanufacturable method of making gapless microlens arrays.

A conceptually simple method for forming gapless microlens arrays is tostamp the profile into a soft material using a rigid die. This techniquegoes by several different terms such as embossing, imprinting, andcontact printing depending on the details of how it is applied. Thistype of technique is used to fabricate micro-optical components forfiber optics and display applications. The standard application involvesa film of material coated on a substrate, which is subsequently stampedwith the die. In most applications either heat or significant pressureis needed to imprint the die image into receiver layer. The applicationof this method to making microlens arrays for electronic image sensorsis not likely since the use of pressure or heat causes distortions.These distortions are not of significant size to effect the quality offiber optic or display devices however the pixel sizes are much smallerfor image sensors and such distortions would severely effectperformance.

Consequently, in view of the above, there is a need for a method tofabricate microlens arrays with reduced or eliminated gaps that is costeffective and produces microlens arrays having minimal distortions.

SUMMARY OF THE INVENTION

The present invention relates to an improved method of forming microlensarrays on electronic image sensors. The improvement involves a methodwhereby adjacent microlenses can be packed close enough together toeliminate any significant gaps between them while allowing the use of apreferred spherical shape. The method involves the use of a templatewith the desired relief image for the microlens array. The imprint stampis brought into contact with a polymerizable fluid composition such thatthe relief image is completely filled with said polymerizable fluidcomposition. The fluid nature of the polymerizable composition andcapillary action allows this relief image filling to be accomplishedwith very little pressure. The imprint stamp is made of a material thatis transparent to the wavelengths of light necessary to photochemicallyharden the polymerizable fluid composition. This allows irradiationthrough the imprint stamp while it is in contact with the polymerizablefluid composition. The result of this irradiation is a hardening of thepolymerizable fluid composition. This hardening permits subsequentremoval of the imprint stamp while the hardened polymerizablecomposition retains the desired microlens shape. The hardenedpolymerizable composition has the necessary optical transmission andstability properties that allow it to be used directly as the microlensarray on electronic image sensors without having to transfer themicrolens shape into an underlying layer by etching techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an image sensor and a template used forcreating microlenses spanning the image sensor which illustrates aninitial step of the present invention in creating microlenses;

FIGS. 2-5 illustrate additional steps of the present invention used increating microlenses spanning the image sensor;

FIG. 6 is a top view of the microlenses formed from the processillustrated in FIGS. 1-5;

FIG. 7 is a top view of microlenses that include overlapping portionscreated by an alternative template using the process of the presentinvention;

FIG. 8 a side view of an alternative template of the present invention;and

FIG. 9 is a side view of the microlens array spanning the image sensorof the alternative embodiment.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment of the present invention, illustrated in FIGS.1-5, a method is provided by which a microlens array is formed onelectronic image sensors. The method provides for adjacent microlensespacked close enough together to eliminate any significant gaps betweenthem while allowing the use of a preferred spherical shape.

Referring to FIG. 1, a semiconductor portion 10, comprising photoactiveareas 12, electrodes 14, and lightshields 16 is shown as representativeof the typical elements of the semiconductor portion of a solid stateelectronic image sensor. For most applications of electronic imagesensors, it is desirable to enhance the characteristics of the incomingelectromagnetic radiation. In order to facilitate this enhancement, aplanarization layer 18 is often applied to the surface of thesemiconductor portion of the electronic image sensor. This planarizationlayer 18 can consist of a variety of materials the only requirementsbeing that it does not adversely affect the spectral characteristics ofthe incoming radiation and is compatible with the materials andprocesses used in the manufacture of electronic image sensors. Since itspurpose is planarization, there must be available a technique wherebythe surface of the planarization layer 18 can be made planar with thesurface of the photoactive areas 12 of the electronic image sensor. Itis possible that simple spin coating would provide a sufficiently planarsurface. However, other techniques such as plasma etch back and chemicalmechanical planarization are commonly available to improve theco-planarity of the surface. Once a planar surface has been achieved, itmay also be desirable to filter the spectral characteristics of theincoming radiation. This is accomplished by applying a color filterlayer 20 consisting of two or more areas of different spectraltransmission patterned so as to be aligned with the underlyingphotoactive areas 12. Since the photoactive areas 12 only comprise aportion of the total electronic image sensor there is a significantamount of incoming radiation that would fall on areas not able tocapture it and produce an electronic signal. This leads to a reductionin the sensitivity of the electronic image sensor so it is oftendesirable to increase the fraction of the incoming radiation that fallson the photoactive areas 12. Fabricating a microlens array on top of theelectronic image sensor whereby the individual microlens elements arealigned with the underlying photoactive areas 12 commonly does this.This microlens array requires both a planar surface and the correctdistance from the surface of the photoactive area to accommodate thefocal distance of the microlenses. These requirements often necessitatethat application of a spacer layer 22 on top of the color filter layer20. Since the spacer layer 22 serves only to physically position themicrolens array, it has similar requirements to the planarization layer18 and is often the same material. The present invention involves animproved method for forming the microlens array. The method involves theuse of a template 30, which consists of a plurality of curved surfacesrepresenting the desired relief image of the microlens array. As shown,the template 30 is aligned over the electronic imager sensor 10 with agap 40.

Referring to FIG. 2, a photopolymerizable fluid composition 50 thencontacts the surface of the spacer layer 22 and the template 30 so as tofill the gap 40 (shown in FIG. 1). The template 30 is made of amaterial, which is transparent to the photoactive wavelengths. Apreferred material for fabricating the template 30 would be quartz,which is both transparent to a wide range of wavelengths and isdimensionally stable. The photopolymerizable fluid composition 50 mayhave a low viscosity such that it may fill the gap in an efficientmanner. Preferably, the viscosity of the photopolymerizable fluidcomposition ranges from about 0.01 cps to about 100 cps measured at 25°C., and more preferably from about 0.01 cps to about 1 cps measured atthis temperature.

Referring to FIG. 3, the template 30 is then moved closer to the spacerlayer 22 to expel excess photopolymerizable fluid composition 50 suchthat the edges of the template 30 come into contact with the spacerlayer 22. The photopolymerizable fluid composition 50 is then exposed toelectromagnetic radiation of appropriate wavelength to polymerize thefluid.

Now referring to FIG. 4, preferably, the photopolymerizable fluidcomposition 50 is exposed to radiation sufficient to polymerize thefluid composition and form a solidified polymeric material representedby 60. Preferably, the photopolymerizable fluid composition is exposedwith ultra violet light, although other means of polymerizing the fluidcomposition are available such as heat or other forms of radiation.

The template 30 then leaves the solidified polymeric material 60 on thespacer layer 22, as shown in FIG. 5. The solidified polymeric material60 is left in the desired microlens shape. Preferably, the solidifiedpolymeric material 60 would have characteristics consistent withfunctioning as a microlens elements (the combination of the microlenselements forms a microlens array) for electronic image sensors. Thesecharacteristics would include transparency to visible wavelengths thatwould not deteriorate with exposure to visible light or heat. Also,these characteristics include a Tg high enough so that the preferredmicrolens shape is preserved during any subsequent operations such asmounting the electronic image sensor in a suitable package.

The microlens array depicted in FIG. 5 has the individual microlensarray elements in close proximity to each other. In this lateral view itwould seem that this is a very efficient arrangement. If, however, theoverhead view of this same microlens array is examined, as shown in FIG.6, it becomes obvious that a significant amount of open space is stillpresent between diagonally adjacent microlens array elements. This typeof microlens array is achievable in the prior art using the techniquesdescribed in the background. The present invention produces a similarmicrolens array using the process described above. Since it isadvantageous to preserve a spherical shape for the microlenses in orderto maximize the light focusing efficiency, increasing the diameter suchthat the diagonally adjacent microlens array elements come in contact ornearly so results in significant overlap of horizontally and verticallyadjacent microlens array elements. This is depicted in an overhead viewin FIG. 7. This close-packed arrangement of microlens array elements isnot possible with the prior art technique involving the melting ofphotoresist patterns. This is because any contact of adjacentphotoresist features during the melting will result in the featuresflowing together thus losing the desired microlens shape.

Referring to FIGS. 7, 8 and 9, the microlens array 70 shown in FIG. 7 iscreated by leaving the center of the individual microlens elements 60 inthe same position over the photoactive areas 12, and expanding theirdiameter such that the gaps between diagonally adjacent microlensesreduce to essentially zero.

The only modification necessary to achieve the microlens array patternshown in FIG. 7 is to change the layout of the microlens array elementsin the template 30 (a template that does not create any gaps betweenadjacent microlenses or that creates some overlap in adjacentmicrolenses). FIG. 8 shows the lateral view of the template needed forthis close-packed microlens array shown in FIG. 7. The processing stepsshown in FIG. 1-5 are followed the same way and result in the electronicimage sensor shown in a lateral cross-section in FIG. 9.

The invention has been described with reference to a preferredembodiment. However, it will be appreciated that variations andmodifications can be effected by a person of ordinary skill in the artwithout departing from the scope of the invention.

Parts List

-   10 semiconductor portion/electronic imager sensor-   12 photoactive areas-   14 electrodes-   16 lightshield-   18 planarization layer-   20 color filter array-   22 spacer layer-   30 template-   40 gap-   50 photopolymerizable fluid composition-   60 solidified polymeric material-   70 microlens array

1. A method for creating microlenses, the method comprising the stepsof: (a) providing a substrate having a plurality of photoactive areas;(b) providing a photopolymerizable fluid composition on the substrate;(c) providing a template which is transparent to photoactive wavelengthsand which includes a plurality of curved surfaces that act to focusincident light onto the photoactive area; (d) placing the template onthe photopolymerizable fluid composition which causes the fluid materialto spread and substantially fill the curved surfaces of the template;and (e) irradiating light through the template and onto thephotopolymerizable fluid composition for hardening photopolymerizablefluid composition into microlenses spanning the substrate.
 2. The methodas in claim 1 further comprising the step of removing the template whichresults in retaining the hardened material on the substrate.
 3. Themethod as in claim 1 further comprising the step of providing a templatethat does not create any gaps between adjacent microlenses or thatcreates some overlap in adjacent microlenses.
 4. The method as in claim1 further comprising the step of providing quartz as the material forthe template.
 5. The method as in claim 1 further comprising the step ofproviding the hardened photopolymerizable fluid composition as havingtransparency to visible wavelengths that will not deteriorate uponexposure to visible light or heat.
 6. The method as in claim 1 furthercomprising the step of providing the hardened photopolymerizable fluidcomposition as having a Tg sufficiently high to preserve a predeterminedmicrolens shape during any subsequent packaging operation.